Cytochrome P-450 comprises a large super family of hemoproteins that collectively catalyze the metabolism of literally thousands of different compounds, including steroids, fatty acids, drugs and carcinogens. Although the general sequences of events that occur during the action of these enzymes are understood, and are common to all members of the family, the mechanisms by which a substrate is converted to several metabolites by a cytochrome P-450 enzyme is not clearly understood. We have derived rate equations for four different plausible mechanisms, namely the rapid and slow interconvertible mechanisms, the parallel pathway mechanism and the dissociative mechanism. Inspection of these equations reveal that a combination of competitive and noncompetitive deuterium isotope effect experiments can differen-tiate these mechanisms. Studies with P-450 2CII reveal that of 2-alpha-hydrogen and 16-alpha-hydroxytestosterone are formed by this isozyme by the dissociative mechanism.